Methods for producing superabsorbent polymers for use in agricultural applications

ABSTRACT

Methods and systems for producing superabsorbent polymer particles for use in agricultural applications are disclosed. A monomer is graft polymerized onto a starch to form a starch graft copolymer. The moisture content of the starch graft copolymer is reduced through the use of an extruder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application Ser. No. 60/762,098, filed Jan. 25, 2006,and titled “Improved Drying Methods For Producing SuperabsorbentPolymers,” which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to superabsorbent polymer products and tonovel methods for producing superabsorbent polymer products.

BRIEF DESCRIPTION OF THE DRAWINGS

The present embodiments will become more fully apparent from thefollowing description and appended claims, taken in conjunction with theaccompanying drawings. Understanding that the accompanying drawingsdepict only typical embodiments, and are, therefore, not to beconsidered to be limiting of the scope of the present disclosure, theembodiments will be described and explained with specificity and detailin reference to the accompanying drawings as provided below.

FIG. 1 is a flow diagram illustrating one exemplary embodiment of amethod for producing starch graft copolymer superabsorbent polymerproducts.

FIG. 2 is a partially cut away side elevation cross sectional viewcombined with a block view of one embodiment of a system used in themanufacture of superabsorbent polymer products.

FIG. 3 is a partially cut away side elevation view of various segmentsof one exemplary embodiment of an extrusion screw.

FIG. 4A is a partially cut away side elevation cross sectional view ofone embodiment of a single screw extruder.

FIG. 4B is a partially cut away side elevation view of one embodiment ofa double screw extruder.

FIG. 4C is a partially cut away side elevation view of one embodiment ofa multi-screw extruder.

FIG. 5A is a bar graph of moisture content of a superabsorbent polymerdough at various passes through an extruder operating at 30 rpm.

FIG. 5B is a bar graph of moisture content of a superabsorbent polymerdough at various passes through an extruder operating between 20 and 25rpm.

DETAILED DESCRIPTION

Those skilled in the art will recognize that the methods, components andcompositions generally disclosed and illustrated in the Figures hereinmay be arranged and practiced in a wide variety of differentconfigurations, such as without one or more of the specific detailsdescribed, or with other methods, components, materials, etc. In somecases, well-known materials, components or method steps are not shown ordescribed in detail. Furthermore, the described components, methodsteps, compositions, etc., may be combined in any suitable manner in oneor more embodiments. Thus, the following more detailed description ofvarious embodiments, as disclosed and represented in the Figures, is notintended to limit the scope of the present disclosure, but is merelyrepresentative of certain exemplary embodiments. While the variousaspects of the embodiments are presented in drawings, the drawings arenot necessarily drawn to scale unless specifically indicated.

The phrases “connected to,” “coupled to” and “in communication with”refer to any form of interaction between two or more entities, includingmechanical, electrical, magnetic, electromagnetic, fluid, and thermalinteraction. Two components may be coupled to each other even thoughthey are not in direct contact with each other.

Superabsorbent polymers (“SAPs”) are materials that imbibe or absorb atleast 10 times their own weight in aqueous fluid and that retain theimbibed or absorbed aqueous fluid under moderate pressure. The imbibedor absorbed aqueous fluid is taken into the molecular structure of theSAP rather than being contained in pores from which the fluid could beeliminated by squeezing. Some SAPs can absorb up to, or more than, 1,000times their weight in aqueous fluid. In one embodiment, SAPs can absorbbetween 200 to 600 times their weight in aqueous fluid.

SAPs may be used in agricultural or horticultural applications. Theterms “agricultural” and “horticultural” are used synonymously andinterchangeably throughout the present disclosure. Applying SAPs to soilin agricultural settings have resulted in earlier seed germinationand/or blooming, decreased irrigation requirements, increasedpropagation, increased crop growth and production, increased cropquality, decreased soil crusting, increased yield and decreased time ofemergence.

Synthetic SAPs are commercially available and are conventionally used inconjunction with baby or adult diapers, catamenials, hospital bed pads,cable coating and the like. However synthetic SAPs may also be used inagricultural applications. Another type of SAP product used more widelyin agricultural applications includes starch graft copolymers. Starchgraft copolymers comprise a monomer graft polymerized onto apolysaccharide, such as a starch or cellulose. Starch graft copolymersare typically used to absorb aqueous fluids for use in absorbentsoftgoods, in increasing the water holding capacity of soils, and ascoatings onto seeds, fibers, clays, and the like.

FIG. 1 is a flow diagram illustrating one exemplary embodiment of amethod 100 for producing SAP particles that may be used in agriculturalapplications. The method 100 illustrated herein provides a method forproducing starch graft copolymer SAP particles. However, it should beappreciated that synthetic polymers may also be used when appropriate asunderstood by one having skill in the art with the aid of the presentdisclosure.

The method 100 of producing a starch graft copolymer SAP for use inagricultural applications involves graft polymerizing 102 a monomer,such as acrylonitrile, onto a polysaccharide, such as starch, in thepresence of an initiator, such as a ceric (+4) salt, to form the starchgraft copolymer. Polymerization 102 may be accomplished over severalminutes producing long grafted chains of polyacrylonitrile, orpolyacrylonitrile combined with other monomers.

Exemplary polysaccharides include cellulose, starches, flours, andmeals. Exemplary starches include native starches (e.g., corn starch(Pure Food Powder, manufactured by A.E. Staley), waxy maize starch (Waxy7350, manufactured by A.E. Staley), wheat starch (Midsol 50,manufactured by Midwest Grain Products), potato starch (Avebe,manufactured by A.E. Staley)), dextrin starches (e.g., Stadex 9,manufactured by A.E. Staley), dextran starches (e.g., Grade 2P,manufactured by Pharmachem Corp.), corn meal, peeled yucca root,unpeeled yucca root, oat flour, banana flour, and tapioca flour. Thestarch may be gelatinized to provide optimal absorbency. An exemplarystarch is gelatinized cornstarch. Furthermore, according to oneembodiment, the weight ratio of the starch to the monomer is in therange of between about 1:1 and about 1:6.

Exemplary initiators for graft polymerizing a monomer onto a starchinclude cerium (+4) salts, such as ceric ammonium nitrate; ammoniumpersulfate; sodium persulfate; potassium persulfate; ferrous peroxide;ferrous ammonium sulfate-hydrogen peroxide; L-ascorbic acid; andpotassium permanganate-ascorbic acid. Other suitable initiators known tothose skilled in the art may be used, such as alternative persulfatesand peroxides, as well as vanadium, manganese, etc. The amount ofinitiator used may vary based on the chosen initiator, the selectedmonomer, and the chosen starch. Some initiators, e.g., persulfates, mayrequire the presence of heat. The initiator may be added in a single ormultiple steps, and multiple initiators may be used.

The resulting starch graft copolymer may be saponified 104 with analkali metal, such as potassium hydroxide or sodium hydroxide, toconvert the nitrile groups into a mixture of carboxamides and alkalicarboxylates. The saponification step may provide a viscous mass ordough.

In alternative embodiments, a monomer, other than acrylonitrile, may begraft polymerized 102 onto a starch in the presence of an initiator toform a starch graft copolymer. Exemplary alternative monomers includeacrylic acid or methacrylic acid. Exemplary monomers may also includeacrylamide or methacrylamide. Sulfonic acids, such as2-acrylamido-2-methyl-propanesulfonic acid (AMPS) and vinyl sulfonicacid may also be used. Moreover, acrylates, such as ethyl acrylate andpotassium acrylate may also be used. Derivatives and mixtures of theabove-listed monomers may also be desirable.

In applications using acrylic acid, the addition of acrylamide theretohelps induce graft polymerization and adds to the absorbency of the SAP.By way of example, the ratio by weight of acrylic acid to acrylamide maybe about 2:1. Alternatively, the ratio of acrylic acid to acrylamide mayalso range up to a ratio of 9:1 and beyond. Because acrylamide isconsidered a neurotoxin, it may be desirable to reduce the relativeamount of acrylamide to acrylic acid, while using enough to help inducegraft polymerization of acrylic acid.

In alternative applications, acrylic acid may graft polymerize onto astarch or other polysaccharide without the assistance of acrylamide. Forexample, acrylic acid may polymerize when placed under heat and/orpressure. Polymerization without the addition of acrylamide may beaccomplished, for example, in a heated screw extruder, such as a singlescrew or a double screw extruder as will be described herein.

In this alternative embodiment, the monomer may be graft polymerizedonto a polysaccharide in the presence of an initiator to form a starchgraft copolymer. Exemplary starches and initiators have been describedabove. The starch graft copolymer may then be cross-linked, for example,by adding a chemical cross-linking agent to form a cross-linked starchgraft copolymer. It may be desirable for the starch graft copolymer tobe cross-linked if it dissolves in aqueous fluids previous to beingcross-linked. Cross-linking is one method to permit the starch graftcopolymer to absorb aqueous fluids without dissolving. However, theamount of cross-linking agent added is typically indirectly proportionalto the absorbency of the resulting SAP product.

Exemplary cross-linking agents include: glycerides; diepoxides;diglycidyls; cyclohexadiamide; methylene bis-acrylamide;bis-hydroxyalkylamides, such as bis-hydroxypropyl adipamide;formaldehydes, such as urea-formaldehyde and melamine-formaldehyderesins; isocyanates including di- or tri-isocyanates; epoxy resins,typically in the presence of a base catalyst; and derivatives andmixtures thereof.

Alternative methods of cross-linking may also be employed. For example,a solid SAP product may be cross-linked through irradiation, such asthrough exposure to gamma or x-ray electromagnetic radiation, or to anelectron beam and the like. Irradiation facilitates cross-linking of thestarch graft copolymer by creating free radicals in the copolymer chain.In some applications, after irradiation an annealing or melting processmay be used to re-form the cross-linked copolymer chains. Furthermore,it may be desirable to perform the irradiation process in an atmosphererelatively free of oxygen.

Although the addition of cross-linking agents may be desirable in theproduction of SAPs, self-cross-linking copolymers may also be used. In aself-cross-linking copolymer, either a single self-reactive functionalgroup or multiple self-reactive functional groups or multipleco-reactive functional groups are incorporated into the mixture. Oneexemplary co-reactive functional group is a copolymer of acrylic acidand glycidyl methacrylate.

Referring to the exemplary embodiment of FIG. 1, the pH of the starchgraft copolymer may be adjusted 106 to a desired value for theparticular agricultural application. For example, the starch graftcopolymer may be neutralized. Alternative pH values may be desirabledepending upon the type of soil and the type of crop the resulting SAPswill be applied to. The resulting pH for most agricultural applicationstypically will range from about 6.0 to about 8.0. The desired pH may begreater or less than this range depending on the requirements for theparticular agricultural application.

Alternatively, in some embodiments, pH adjustment of the starch graftcopolymer may occur earlier, such as prior to the cross-linking stepsummarized in the alternative method described above. In alternativeembodiments, pH adjustment may not be necessary. For instance, ifpotassium acrylate were used as the monomer, the resulting product mayalready be within an acceptable pH range.

In conventional systems, after the starch graft copolymer is saponified104, the isolated product is recovered from the viscous polymerizationdough with the use of water miscible solvents such as alcohols.Exemplary alcohols for use with this conventional method includemethanol, ethanol, propanol and isopropanol. Methanol is typically usedto remove water content and the product is subsequently dried.

However, according to the methods of the present disclosure, moisturewithin the saponified starch-based polymeric dough can be removed 108through the use of an extruder. Using extrusion technology, thepolymerization dough, which may be approximately 80% moisture by weight,can be dried and isolated without the aid of water miscible solvents,such as methanol.

After saponification 104 and optional pH adjustment 106, thepolymerization dough may be introduced to an extruder, such as a singlescrew, double screw or multi-screw extruder. Extrusion technology isknown to those having skill in the art, and exemplary extruders that maybe used for drying the polymerization dough may be purchased from ENTEKManufacturing Inc. of Lebanon, Oreg. Exemplary extruders may employvariously sized screw arms, such as screws having a diameter rangingfrom about 27 mm (or less) to about 133 mm (or more). The barrels withinwhich the extrusion screw(s) auger the polymerization dough may alsovary in length, and may range from about 110 mm (or less) to about 510mm (or more).

In one exemplary embodiment, the polymerization dough havingapproximately 80% moisture content by weight may be dried to about 40%moisture content by weight after extrusion. Moisture content representsthe amount (percentage) of aqueous fluid by weight in the polymerizationdough. In another embodiment, the moisture content may be reduced to anamount below about 65% by weight. In yet another embodiment, themoisture content may be reduced to an amount less than or equal to about40% by weight. All moisture content may be removed via an extruder ifdesirable. As will be described in greater detail below, the moisturecontent is dependent upon a number of variables, such as number ofbarrels, barrel length, temperature, rpm of the extrusion screw(s),number of passes through the extruder and other factors which will bediscussed at greater length below.

During the drying process the SAP product may comprise brittle particlesbecause of the application of heat and the resulting loss of moisture.Depending on the aforementioned variables affecting resulting moisturecontent, the physical characteristics of the SAP product exiting theextruder may vary. The moisture-reduced SAP particles may be dischargedby the extruder to be sized and packaged.

Upon exiting the extruder, the moisture-reduced SAP product may bewetted 110 with methanol, or another water-miscible solvent discussedabove, to prevent the SAP granules from sticking together. A smallamount of methanol may be lightly sprayed on the SAP granules to preventre-agglomeration of the particles. Alternatively, a dusting agent may beapplied 111 to the SAP product to minimize re-agglomeration of thegranules. Coating the SAP product with a dusting agent decreases theirpropensity to stick together. Exemplary dusting agents includecellulose, clay, starch, flour, and other natural or synthetic polymersthat prevent the granules from sticking together.

The size of the moisture-reduced SAP product exiting the extruder may beaffected by several variables, such as the size of the holes in the dieplate, the speed of the extrusion screw, the moisture content of thepolymerization dough introduced to the extruder, etc. After passingthrough the extruder, the SAP product may be introduced to a grinder,chopper or granulator and subsequently granulized 112 or pelletized. TheSAP product may also optionally be exposed to water miscible solventssuch as alcohol, e.g., methanol. The resulting dough may be immersedinto the alcohol, and the alkali starch graft copolymer is precipitatedinto particles while being granulated. Using the extruder to remove 108moisture requires less methanol than conventional systems in thisoptional step.

The moisture-reduced SAP particles may be further dried 113. A dryer maybe employed to remove any additional moisture as desired. The dried SAPparticles may also be screened 114 based on size through a particleseparation or screening system to separate out SAP particles havingcommon mesh sizes. Various particle separation systems may be used aswould be apparent to those having skill in the art with the aid of thepresent disclosure.

Depending on the agricultural application, the final SAP product mayhave a particle size that is courser than about 300 mesh. For example,in some applications where the starch graft copolymer is applieddirectly into the soil with the crop, the particle size is courser thanabout 50 mesh, such as between about 8 to about 25 mesh. This particlesize range correlates to commercially available granule applicators.Therefore, alternative particle sizes may be used.

Finer particle sizes are typically used in seed coating or root dippingapplications. By way of example, the particle size for seed coating maybe between about 75 and about 300 mesh, such as about 100 mesh. For rootcoating, the particle size may be between about 30 mesh and about 100mesh, such as about 50 mesh.

By drying the SAP product in an extruder instead of through conventionalmethods, the environmental problems associated with handling toxicalcohols, such as methanol, are minimized. Furthermore, methanol (andother alcohols) is flammable and may pose a fire hazard. Using extrusiontechnology instead of methanol to remove moisture from thepolymerization dough reduces this hazard. Moreover, methanol is costlyto dispense and to purchase. Therefore, drying the SAP product in anextruder also provides a cost savings.

During the process of producing the starch graft copolymer dough,various additives may optionally be included at different stages duringproduction of the SAP product. For example, additives to promote plantgrowth may be included at some stage of the SAP production process, suchas previous to drying, as would be apparent to those having skill in theart with the aid of the present disclosure. One exemplary additiveincludes fertilizer. Various fertilizers that are commercially availablemay be included. In some embodiments, controlled-release fertilizers maybe used. Alternative or additional additives that may also be includedare, without limitation, pesticides, herbicides, fungicides, growthhormones and regulators, mycorrhizal fungi, kelp products, soil-basednutrients and the like.

Exemplary pesticides that may be added include, but are not limited to,acaricides, algicides, antifeedants, avicides, bactericides, birdrepellents, chemosterilants, herbicide safeners, insect attractants,insect repellents, insecticides, mammal repellents, mating disruptors,molluscicides, nematicides, plant activators, plant-growth regulators,rodenticides, synergists, and virucides. Exemplary microbial pesticidesinclude bacillus thuringiensis and mycorrhizal fungi. Exemplaryinsecticides include, but are not limited to, thiodan, diazinon, andmalathion.

Exemplary commercially available pesticides include, but are not limitedto: Admire™ (imidacloprid) manufactured by Bayer, Regent™ (fipronil)manufactured by BASF, Dursban™ (chlorpyrifos) manufactured by Dow,Cruiser™ (thiamethoxam) manufactured by Syngenta, Karate™(lambda-cyhalothrin) manufactured by Syngenta, and Decis™ (deltamethrin)manufactured by Bayer. A combination or blend of pesticides may also beused. Alternative pesticides may also be used as would be apparent tothose having skill in the art with the aid of the present disclosure.

Fungicides may also be included with the SAP product during or afterproduction. Fungicides may help control or prevent the growth of mold orfungus on the roots, seeds or seedlings thus inhibiting root or seedrot. Exemplary commercially available fungicides include, but are notlimited to: Amistar™ (azoxystrobin) manufactured by Syngenta, Folicur™(tebuconazole) manufactured by Bayer, OPUS™ (epoxiconazole) manufacturedby BASF, Dithane™ (mancozeb) manufactured by Dow, Flint™(trifloxystrobin) manufactured by Bayer, and Ridomil™ (metalaxyl)manufactured by Syngenta. A combination or blend of fungicides may alsobe used. Alternative fungicides may also be used as would be apparent tothose having skill in the art with the aid of the present disclosure.

Exemplary commercially available herbicides that may be added before orafter production of the SAP product include, but are not limited to:Roundup™ (glyphosate) manufactured by Monsanto, Gramoxone™ (paraquat)manufactured by Syngenta, Harness™ (acetochlor) manufactured byMonsanto, Prowl™ (pendimethalin) manufactured by BASF, Dual™(metolachlor) manufactured by Syngenta, and Puma™ (fenoxaprop)manufactured by Bayer. Furthermore, a combination or blend of herbicidesmay be used. Alternative herbicides may also be used as would beapparent to those having skill in the art with the aid of the presentdisclosure.

Exemplary commercially available plant-growth regulators that may beoptionally added during or after production of the SAP product include,but are not limited to: Ethrel™ (ethephon) manufactured by Bayer, Pix™(mepiquat) manufactured by BASF, Dropp™ (thidiazuron) manufactured byBayer, Finish™ (cyclanilide) manufactured by Bayer, and Royal MH™(maleic hydrazide) manufactured by Crompton. A combination or blend ofgrowth regulators may be used. Furthermore, growth inhibitors, growthretardants, growth stimulants, and derivatives and mixtures thereof maybe included. Alternative growth regulators or hormones may also be usedas would be apparent to those having skill in the art with the aid ofthe present disclosure.

Exemplary soil-based nutrients that may also optionally be added duringor after production of the SAP product include calcium, magnesium,potassium, phosphorus, boron, zinc, manganese, copper, iron, sulfur,nitrogen, molybdenum, silicon, ammonium phosphate, fish meal, organiccompounds and additives, organic based fertilizers derived from plantand animal products and derivatives, blends, and mixtures thereof. Moreinformation about exemplary growth-promoting additives can be found inThe Farm Chemicals Handbook published by Meister Publishing Company.

FIG. 2 represents one embodiment of an exemplary manufacturing systemthat may be used with the methods described herein to produce SAPparticles. After the starch graft copolymer is saponified and optionallytitrated, the viscous polymerization dough is placed in a hopper 220,which feeds the polymerization dough into an extruder 222. In oneembodiment, the polymerization dough may be introduced to the extruder222 via an auger feed leading into a progressive cavity pump.

In one embodiment, the flow of polymerization dough is consistently feedinto the extruder 222 via the hopper 220. The hopper 220 may include apump feeding system such that the polymerization dough enters theextruder 222 under pressure. A pump feeding system may minimize airbubbles, which might otherwise form in the polymerization dough. In oneembodiment, the polymerization dough may be under a pressure rangingfrom 25 psi to about 40 psi. Alternatively, the polymerization dough maybe exposed to atmospheric pressure. Alternative pressure levels may beapplied as would be apparent to those having skill in the art with theaid of the present disclosure, such as when the extruder speed isvaried.

Furthermore, the polymerization dough may be introduced to the extruder222 at an elevated temperature, such as between about 30° C. and about100° C., such as about 50° C. The polymerization dough may also beintroduced to the extruder 222 at various viscosities. For example, inone embodiment the viscosity of the polymerization dough may range frombetween 2.5 million centipoise and 0.5 million centipoise. In anotherembodiment, the viscosity of the polymerization dough entering theextruder 222 may be about 2.0 million centipoise.

The extruder 222 may comprise a series of barrels 224 with at least oneextrusion screw 226 disposed axially therein. The extrusion screw 226may be driven by screw drive motor 227. Since various sizes of extruders222 may be used, the motor size may also vary. In one embodiment asmaller 5 horsepower motor may be used. In another embodiment a 50horsepower motor may be used. The type of motor is typically dependentupon the size of the extruder 222. The series of barrels 224 may includea feed barrel 228 that receives the polymerization dough from the hopper220. The rotation of the extrusion screw 226 causes the polymerizationdough to travel down the length of the series of barrels 224.

By way of example, between 8 to 24 barrels 224 may be arranged serially.Alternatively, between 20 to 24 barrels 224 may be arranged serially. Inanother embodiment, between 16 to 20 barrels 224 may be arrangedserially. In yet another embodiment, between 8 to 10 barrels 224 may bearranged serially. It has been found that the greater number of barrelsused, the greater the drying effect the extruder 222 has in a singlepass. The drying effect is of course dependent upon a number of othervariables, such as screw speed, temperature, venting, etc. In anembodiment where fewer barrels 224 are used, the polymerization doughmay be passed through the extruder 222 multiple times in order toachieve a desired moisture content. Moreover, more than one extruder 222may be used in series.

Each barrel 224 provides an environment to the polymerization dough thatpasses there through. The environment from barrel to barrel may vary.For example, heat may be applied to the polymerization dough while in agiven barrel 224 to aid in removing moisture from the dough. Convectiveheat may be transferred to the dough from heating coils or other heatingdevices within or adjacent to the barrel walls. Alternatively, a barrel224 may be cooled to cool the polymerization dough passing therethrough. Cooling the polymerization dough may be desirable to drawmoisture to the outer surface of the dough.

In one embodiment, all barrels, with the exception of the feed barrel228 are heated. In another embodiment, every four or five barrels 224are heated, followed by two cooling barrels. Different configurationsmay be used as desired. In one embodiment, the temperature of a givenheating barrel 224 is set at a temperature that ranges from about 50° C.to about 275° C. In another embodiment, the barrels 224 may be set at atemperature ranging from about 50° C. to about 160° C. In yet anotherembodiment, the barrels 224 may be set at a temperature ranging fromabout 125° C. to about 150° C. While higher temperatures may dry thepolymerization dough more quickly, it has been found that at highertemperatures the polymerization dough may plasticize. Accordingly, inanother embodiment the barrels 224 may be set at a temperature of about130° C. As noted above, however, the temperature settings may vary frombarrel to barrel, as desired.

Mechanical heat may also be generated within the barrel of the extruderbased on screw configurations. For example, compression screws, reverseelements, shear locks, etc., can all generate mechanical heat that mayfurther aid in removing moisture from the polymerization dough.

Furthermore, multi-stage vacuum chambers and atmospheric vents may alsoaid in the drying process. In one embodiment, a barrel 224 may include avent 230, from which moisture may escape the heated extruder 222. Avacuum source 232 may optionally be coupled to vent 230 to apply anegative pressure facilitating the extraction of steam being produced bythe heat of the barrels 224. According to one embodiment, approximatelyhalf of the barrels 224 may include vents 230, optionally coupled to avacuum 232.

The speed of the extrusion screw 226 may vary or remain constant duringa particular run. In one embodiment, the speed of the extrusion screw226 may operate between about 20 rpm to about 150 rpm. In anotherembodiment the speed of the extrusion screw may operate between about 80rpm and about 150 rpm. In yet another embodiment the speed of theextrusion screw may operate between about 20 rpm and about 30 rpm. Inanother embodiment the speed of the extrusion screw may operate betweenabout 50 rpm and about 80 rpm. The speed of the extrusion screw effectsthe residence time the polymerization dough is spent in each barrel 224.

A die plate 234 may be located at the outlet of the extruder 222 tocreate a desired size and shape of the SAP product. The die plate 234may help to maintain desirable size and shape characteristics of thepolymerization dough. In one embodiment, the die plate 234 may createrod-shaped forms of the SAP product as the moisture-reduced SAPparticles are pushed there through. Various other shapes of SAP productmay be used as desirable. Selecting an appropriate die can vary therod-shaped forms.

The die plate 234 may comprise a plate that has been drilled or formedto contain holes of a particular size and shape. For example, thediameter of the rods may be controlled by the diameter of the holesdrilled in the end plate. According to one embodiment, the holes in theend plate may range from between about 1/16 inch to about ½ inch indiameter or greater.

The throughput of the polymerization dough out of the extruder 222 mayvary. For example, only a few pounds per hour may be passed through theextruder 222, or alternatively thousands of pounds per hour ofpolymerization dough may pass there through.

The moisture-reduced SAP product may be deposited onto an air-cooledconveyer 236 after passing through the die plate 234. The air-cooledconveyer 236 helps the drying process by drawing moisture from withinthe polymerization dough to the outer surface. Therefore, subsequentdrying methods may be more effective in removing moisture when themoisture has been drawn to the surface. This may be particularly truewhen the polymerization dough is passed through the extruder 222 for asecond time.

As discussed above, before the moisture-reduced SAP product is conveyedto the granulator 238, the SAP product may be wetted with methanol toprevent re-agglomeration of the particles. Alternatively, a dustingagent may be applied to decrease the propensity of the moisture-reducedSAP product to stick together. An in-line granulator 238 may receive themoisture-reduced polymerization dough to granulate the SAP product. Thegranulated SAP particles may subsequently be centrifuged in a centrifuge240. Alternatively, the SAP particles may be decanted through decantingtechnology and methods known to those having skill in the art. A dryer242 may also be employed to remove additional moisture as desired. Inone embodiment, a final moisture content of approximately 12% by weightor less is desirable.

The dried SAP particles made by the above methods may then be passedthrough a particle separation system 244 such as, for example, ascreening system comprising an 8 mesh screen, followed by a 25 meshscreen, followed by a 60 mesh screen, followed by a 100 mesh screen anda fines collection pan. Alternatively and by way of example, particleseparation systems sold under the brand ROTEX® may be used.

FIG. 3 shows one exemplary embodiment of an extrusion screw 326 from apartially cut away side elevation view. The extrusion screw 326 maycomprise multiple segments 350. Each segment may correspond with abarrel, and may perform a particular function. For instance, the flightsof each segment 350 may differ from segment to segment. Alternatively, asingle segment may have multiple flights, such that multiple actions maybe performed in a single barrel. One exemplary flight may knead thepolymerization dough that passes there through. Another flight mayfacilitate the passage of the dough over a particular time interval, toensure a particular exposure to a given barrel environment. Yet anotherflight may chop the polymerization dough into pieces.

FIGS. 4A through 4C show various embodiments of heated screw extrudersfrom a partially cut away side elevation view. FIG. 4A shows oneembodiment of an extruder 422 a with a single extrusion screw 426 a.FIG. 4B illustrates another embodiment of an extruder 422 b with adouble extrusion screw 426 b. With a double screw extruder, the screwsintermesh as they turn and the polymerization dough is worked into acontinuous dough thereby. FIG. 4C demonstrates a multi-screw extrusionsystem employing more than two extrusion screws 426 c in a singleextruder 422 c as would be apparent to those having skill in the artwith the aid of the present disclosure.

Because the operating parameters and configuration of the extruder isselected to maximize SAP product production and performance in varioussettings, the configuration and operating parameters may vary greatly.Therefore, the following examples are intended to further illustrateexemplary embodiments, and are not intended to limit the scope of thedisclosure.

EXAMPLE 1

A 13-barrel double-screw extruder comprising one feed barrel and twelveheated barrels were used in series. Barrel number 5 along the seriesincluded an atmospheric vent. Barrel number 7 included a vent coupled toa vacuum pump. Barrel numbers 9 and 11 also included an atmosphericvent. A pump feeding system was also used to apply greater thanatmospheric pressure on the polymerization dough. The polymerizationdough was deposited on an air-cooled conveyer upon exiting the extruder.The polymerization dough was subsequently placed back into the feedbarrel for an additional pass.

The temperature settings of the barrels were kept constant, and themoisture content was measured after each pass through the extruder. Thepressure was varied slightly and the speed of the extrusion screw wasvaried as detailed in Table 1 below. The initial moisture content of thepolymerization dough was approximately 79%. As noted below, only twopasses through the extruder resulted in a moisture content of less than40%.

TABLE 1 Temp. Speed Pressure Moisture Pass # (° C.) (rpm) (psi) content1 150 80 27 46.37% 2 150 80 25 39.14% 3 150 30 25 35.71% 4 150 30 2524.82%

EXAMPLE 2

A 13-barrel double-screw extruder comprising one feed barrel and twelveheated barrels were used in series. Barrel number 5 along the seriesincluded an atmospheric vent. Barrel number 7 included a vent coupled toa vacuum pump. Barrel numbers 9 and 11 also included an atmosphericvent. A pump feeding system was also used to apply greater thanatmospheric pressure on the polymerization dough. The polymerizationdough was deposited on an air-cooled conveyer upon exiting the extruder.The polymerization dough was subsequently placed back into the feedbarrel for an additional pass.

The temperature settings of the barrels were kept constant at 150° C.,and the moisture content was measured after each pass through theextruder. The speed of the extrusion screw was also kept constant at 30rpm. The initial moisture content of the polymerization dough wasapproximately 82.2%. The results were detailed in Table 2 below.

TABLE 2 Moisture Pass # content 1 64.82% 2 58.76% 3 48.73% 4 27.30% 522.72%

The moisture trend for each pass of the extruder at a temperature of150° C. and an extrusion screw speed of 30 rpm of Table 2 were plottedin the bar graph shown in FIG. 5A.

EXAMPLE 3

A 13-barrel double-screw extruder comprising one feed barrel and twelveheated barrels were used in series. Barrel number 5 along the seriesincluded an atmospheric vent. Barrel number 7 included a vent coupled toa vacuum pump. Barrel numbers 9 and 11 also included an atmosphericvent. A pump feeding system was also used to apply greater thanatmospheric pressure on the polymerization dough. The polymerizationdough was deposited on an air-cooled conveyer upon exiting the extruder.The polymerization dough was subsequently placed back into the feedbarrel for an additional pass.

The temperature settings of the barrels were kept constant at 150° C.,and the moisture content was measured after each pass through theextruder. The speed of the extrusion screw was varied between 20 and 25rpm. The initial moisture content of the polymerization dough wasapproximately 82.2%. The results were detailed in Table 3 below.

TABLE 3 Moisture Pass # content 1 62.04% 2 55.17% 3 43.14% 4 28.43% 534.41%

The moisture trend for each pass of the extruder at a temperature of150° C. and an extrusion screw speed of between 20 and 25 rpm of Table 3were plotted in the bar graph shown in FIG. 5B.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments.Furthermore, the methods disclosed herein comprise one or more steps oractions for performing the described method. The method steps and/oractions may be interchanged with one another. In other words, unless aspecific order of steps or actions is required for proper operation ofthe embodiment, the order and/or use of specific steps and/or actionsmay be modified.

1. A method of producing a superabsorbent polymer product for use inagricultural applications, comprising: graft polymerizing a monomer ontoa starch in the presence of an initiator to form a starch graftcopolymer, the starch graft copolymer forming a polymerization dough;feeding the polymerization dough into a heated extruder comprising anextrusion screw at a pressure of between 25 psi and 40 psi, thepolymerization dough having a viscosity of between 2.5 millioncentipoise and 0.5 million centipoise when introduced to the heatedextruder; operating the extrusion screw at a speed of between about 20rpm to about 150 rpm; reducing the moisture content of thepolymerization dough by drying the polymerization dough in the heatedextruder, wherein the heated portions of the extruder heat thepolymerization dough at a temperature of between 50° C. and 160° C.;cooling the polymerization dough in such a manner to allow moisture tobe drawn to the outer surface of the polymerization dough after exitingthe heated extruder; and drying the polymerization dough a second timeafter cooling the polymerization dough.
 2. The method of claim 1,wherein reducing the moisture content comprises reducing the moisturecontent of the polymerization dough to less than 65% by weight by dryingthe polymerization dough in the heated extruder.
 3. The method of claim1, wherein reducing the moisture content comprises reducing the moisturecontent of the polymerization dough to about 40% by weight or less bydrying the polymerization dough in the heated extruder.
 4. The method ofclaim 3, wherein reducing the moisture content of the polymerizationdough comprises passing the polymerization dough through the heatedextruder multiple times until the moisture content is about 40% byweight or less.
 5. The method of claim 1, further comprising:saponifying the starch graft copolymer before drying the polymerizationdough in the heated extruder; wherein the monomer is at leastacrylonitrile.
 6. The method of claim 1, further comprising: adjusting apH of the starch graft copolymer to between about 6.0 and about 8.0before drying the polymerization dough in the heated extruder.
 7. Themethod of claim 6, further comprising: cross-linking the starch graftcopolymer; wherein the monomer is other than acrylonitrile and themonomer is chosen from: acrylic acid, acrylamide, methacrylamide,2-acrylamido-2-methyl-propanesulfonic acid, methacrylic acid, vinylsulfonic acid, ethyl acrylate, potassium acrylate, and derivatives andmixtures thereof.
 8. The method of claim 1, further comprising:supplying an additive to the polymerization dough, wherein the additiveis chosen from at least one of the following: fertilizers, herbicides,pesticides, fungicides, and growth regulators.
 9. The method of claim 1,wherein reducing the moisture content comprises heating thepolymerization dough in the heated extruder at a temperature betweenabout 125° C. and about 150° C.
 10. The method of claim 1, wherein theextrusion screw is operated at a speed between about 50 rpm and about 80rpm.
 11. The method of claim 1, wherein the heated extruder comprises atleast one vent, the method further comprising: applying a negativepressure to the at least one vent to facilitate the removal of steamfrom the heated extruder.
 12. The method of claim 1, wherein cooling thepolymerization dough further comprises cooling the polymerization doughat an interval within the heated extruder.
 13. The method of claim 1,further comprising: granulating the polymerization dough after themoisture content of the polymerization dough is reduced in the heatedextruder.